The present invention relates to detecting the level of a liquid such as water held in many different kinds of containers. The liquid may be contained under pressure as in the case of nuclear pressure vessels. The liquid level detectors of interest herein have a positive temperature-coefficient of electrical resistivity and are driven by a constant current source. Such detectors may be used for level sensing in boiling water and pressurized water reactors. However, they may also be employed to measure liquid levels in many other kinds of containers.
The detectors themselves can be inserted into their respective containers from above or below, or even through their sides. In the particular case of a pressurized water reactor, the detectors are generally inserted through the top of the pressure vessel. In boiling water reactors, insertion is typically from below, because of a difference in construction.
In nuclear reactors, generally, the pressure vessel typically contains a core of nuclear fuel that is cooled to prevent core degradation. Cooling the nuclear fuel concomitantly heats the water flowing through the core of the reactor. At a sufficiently high temperature, the water changes into steam and may drive a turbine generator to produce electricity.
More details regarding nuclear reactors and power generation are found in Energy Technology Handbook, edited by Douglas M. Considine and published by McGraw Hill Book Company in 1977.
Considering the pressure vessel of a reactor to be positioned vertically, the water level in the pressure vessel tends to vary radically from the center of the vessel depending upon the internal flow of water. For this reason it is desirable to distribute level detectors at various locations throughout the pressure vessel.
In pressurized water reactors the level detectors are particularly useful near the top of the pressure vessel, since these reactors are normally completely filled with water. On the other hand, in boiling water reactors, the normal operating water level extends only a few feet above the core itself. Consequently, the level sensors are primarily useful in that general threshold region.
The level detectors discussed herein resist the flow of electric current. This generates heat and changes the temperature in the detector, depending upon the level of the electric current passing through the detector and the nature of the cooling at the detector surface. When the detector is immersed in a favorable heat dissipation medium such as water, the heat generated tends to dissipate from the detector more rapidly than it would in air or another gaseous environment. The relatively faster dissipation of heat in liquids, specifically water, causes the temperature of the level detector to drop when the level detector is placed in liquid, or when the liquid level rises to cover the detector wholly or partially.
A rise in the temperature of the detector causes its resistance to increase. Table I illustrates the relationship between temperature and resistance changes in the level detector employing a positive temperature coefficient material as a sensing element, which is driven by a constant voltage or, alternatively, by a constant current source.
TABLE I ______________________________________ Changes in temperature .DELTA.T.sub.1 .DELTA.R.sub.1 .DELTA.I .DELTA.T.sub.2 .DELTA.R.sub.2 or resistance (primary and secondary), and in current Detector driven by a + + - - - constant voltage source - - + + + Detector driven by a + + .0. + + constant current source - - .0. - - ______________________________________
For example, with a constant voltage source driving the level detector, the current "I" through the detector increases or decreases by an amount .DELTA.I, depending upon whether the environment of the level detector changes from gas to liquid, or from liquid to gas. For example, when the detector environment changes from liquid to gas, detector resistance increases by an initial amount of .DELTA.R.sub.1 with the increased initial temperature change .DELTA.T.sub.1 caused by decreased heat dissipation. This initial or primary resistance increase .DELTA.R.sub.1 decreases the current through the detector, if it is driven by a constant voltage source, and in turn causes a secondary temperature decrease .DELTA.T.sub.2 due to the diminished heat generation. This results in an overall reduced temperature change .DELTA.T.sub.1 +.DELTA.T.sub.2 and a reduced net resistance .DELTA.R.sub.1 +.DELTA.R.sub.2. Simply stated, this amounts to reduced sensitivity for the level detector, because of its being constant voltage driven.
With a constant current source driving the level detector, the current "I" through the detector does not change with a change of environment. But when, for example, the detector environment changes from liquid to gas, detector resistance increases by an initial amount of .DELTA.R.sub.1, with the increased initial temperature change .DELTA.T.sub.1 caused by decreased heat dissipation. This initial or primary resistance increase .DELTA.R.sub.1 is not accompanied by a decrease in current through the detector, if it is driven by a constant current source, and the result of the same current flowing through a higher resistance is an increase in heat generation accompanied by a temperature increase .DELTA.T.sub.2. This results in an overall increased temperature change .DELTA.T.sub.1 +.DELTA.T.sub.2 and an increased resistance .DELTA.R.sub.1 +.DELTA.R.sub.2. Simply stated, this amounts to increased sensitivity for the level detector, because of its being constant current driven.
In a constant current driven level detector operating with positive temperature coefficient of resistivity sensing materials, there is accordingly an increased sensing capacity caused by an increase .DELTA.R.sub.2 in the primary resistance change .DELTA.R.sub.1. The resistance change .DELTA.R.sub.1 caused by a change of medium in the immediate proximity of the detector (e.g., a drop in water level causing the level sensor to become resident in air or steam) causes no current change with a constant current source. Therefore, secondary effects occur to increase sensitivity.